What led to America's shift from isolationism to interventionism after WWII?

The primary basis for America's transition from historical isolationism to a more interventionist foreign policy after World War II was the fundamental fear of Soviet- style Communism.  This fear is what drove the interventionist attitude that dominated American foreign policy and forged the basis of the Cold War. It was the fear of this dialectical other in a bipolar world that ensured America would assume a more interventionist foreign policy as a response to the Soviet Union.

Upon the conclusion of World War II, Europe was decimated.  The once looming and dominant power of the German nation had been reduced to the very rubble that most of Europe resembled.  There was no real order left in Europe and this created a vacuum Stalin clearly recognized.  As early as the 1920s, Stalin had asserted a fundamental mistrust of Western capitalism and its relationship towards Russia, arguing that "the present capitalist encirclement" must be "replaced by a socialist encirclement."  Even though the Soviet Union signed the Non- aggression pact with the Nazis, when Hitler invaded Poland, it demonstrated to Stalin that some type of buffer between his nation and the Western nations was required.  During World War II, Stalin interpreted the Eastern front as the West's attempt to weaken the Soviet Union by ensuring that the Russians fought the Germans without much in way of assistance, thereby furthering mistrust of the West.  Upon conclusion of the war, Stalin began the process of angling reconstruction towards a position that enabled Russian protection through the establishment of a "socialist encirclement."  At the root of this was a mistrust of the West.

On the American side of the ledger, the death of  President Roosevelt reoriented his global vision for American economic superiority over the British Empire.  Vice President Truman assumed power and displayed an intense mistrust of Stalin. Rather than accept and continue the diplomatic advances that Roosevelt had established, Truman looked at Stalin's desire to install Soviet friendly governments in the nations that bordered the Soviet Union with hostility. Truman and his advisors became convinced that the Soviet Union was seeking to become the new dominant force in Europe.  As the United States was developing nuclear technology, it became clear that the Soviets were also following suit.  Truman understood that the need for the United States to develop such technology was to defeat Japan.  He recognized and interpreted the Soviet desire to compete as a desire to establish unilateral power in both Europe and the world.

Both nations' aggressiveness and hostility towards the other manifested itself in the new map of Europe.  Both the establishment of the Eastern Bloc and Soviet manipulation of governments in Eastern Bloc nations to suppress any dissent through any means necessary demonstrated to Truman and his advisors that Stalin would not be content until he controlled all of Europe. When the issue of Germany arose, the interventionist foreign policy of both nations became clear. Internal documents amongst Soviet advisors suggested that the United States and Western capitalist nations sought to destroy the Soviet Union, through which occupation of Germany was one of the first steps. This necessitated a Soviet military buildup within Germany in order to repel what was seen as the West's desire to destroy the Soviet Union. At the same time, American interests in Germany were demonstrated when the United States established that it would remain in Germany indefinitely, constituting a political and economic presence in the nation in order to provide a fundamental obstacle to Soviet control:  "The nub of our program was to win the German people ... it was a battle between us and Russia over minds ..."  

The need to view Europe in bipolar terms helped to establish a foreign policy approach that ran away from isolationism and into interventionism.  Soviet- style Communism became seen as the greatest threat to American freedom and the condition of newly freed nations around the world.  It was a force that had to be countered with aggressiveness and nothing short of total commitment would do.  The "battle between us and Russia over minds" took on the form of interventionism wherever, whenever, and however it was needed.  Containing Communism became the basis for the transition from isolationism to American interventionism.

Truman's policy of containment bound the United States into an interventionist foreign policy.  The Truman Doctrine was defined through American interventionism against Communism.  In a speech before Congress in 1947 that outlined the foundations of the Truman Doctrine, the President constructed a paradigm where isolationism was viewed as threatening a state of freedom whose presence demanded intervention:

To ensure the peaceful development of nations, free from coercion, the United States has taken a leading part in establishing the United Nations, The United Nations is designed to make possible lasting freedom and independence for all its members. We shall not realize our objectives, however, unless we are willing to help free peoples to maintain their free institutions and their national integrity against aggressive movements that seek to impose upon them totalitarian regimes. This is no more than a frank recognition that totalitarian regimes imposed on free peoples, by direct or indirect aggression, undermine the foundations of international peace and hence the security of the United States.

President Truman's insistence that the United States had a moral and political obligation to "help free peoples to maintain their free institutions and national integrity against aggressive movements that seek to impose upon them totalitarian regimes" established the transition from historical isolationism to a more interventionist foreign policy after World War II.

What led Americans to favor international involvement over isolationism during WWII?

After the conclusion of World War I in 1918, the United States became isolationist for most of the following two decades. The US did not, for example, join the League of Nations. America was preoccupied with domestic matters during the Roaring Twenties and the Great Depression. When World War II erupted in 1939, Washington tried to remain neutral. Although isolationism was strong, President Franklin Roosevelt and most Americans sympathized with the Allies. Roosevelt tried to help Britain a much as possible, short of actual war, especially after the fall of France in June 1940. If Britain were to fall, too, all of Europe would have been under Nazi domination. Therefore, FDR made America "the arsenal of democracy." Nevertheless, Charles Lindbergh and other isolationists were a powerful political force. And America remained neutral, in spite of the considerable aid it provided to Britain.

Events in Asia ultimately led to direct American participation in the war as a belligerent. Japan had been steadily expanding its influence in China and—after the fall of France—elsewhere in Asia. Washington responded with increasingly severe economic sanctions. Japan was being squeezed economically by the sanctions, so it decided it had to attack the United States. On December 7, 1941, Japan attacked Pearl Harbor. Adolph Hitler, who was allied to Japan, then declared war on the United States. Japan's sneak attack ended American isolationism, and the American public was unified and determined to win World War II.